1. Field of the Invention
This invention relates to such image forming apparatuses as, for example, electrophotographic copying machines, printers, and facsimile systems or to a fixing device to be used in such apparatuses. More particularly, it relates to an induction heating type fixing device which, by virtue of an alternating current applied to a coil, a fixing roller made of a conductive material is caused to generate an induced electric current and consequently is heated with the induced electric current.
2. Description of the Related Art
The electrophotographic copying machines, printers, facsimile systems, etc. are provided with the fixing device which fixes on the sheets of such recording media as recording papers or transfer materials a toner image transferred onto the sheets.
The fixing device is provided, for example, with a fixing roller for thermally fusing the toner on the sheet and a pressure roller pressed against the fixing roller and enabled to nip the sheet in cooperation therewith. The fixing roller is formed in a cylindrical shape, provided inside the cylinder with a heating element, and adapted to heat the fixing roller to a temperature necessary for melting and fixing the toner.
Heretofore, the heating elements of this class have utilized predominantly a halogen lamp, for example, which is disposed along the axis of the fixing roller and adapted to heat the fixing roller with the radiant heat thereof. Though the heating elements using the halogen lamp are inexpensive, they suffer from low thermal efficiency and heavy energy loss because the radiant heat is used for the heating.
The fixing devices using the induction heating have been proposed with a view to eliminating such weak points of the heating with the halogen lamp as mentioned above and fulfilling the demand voiced in recent years for conservation of energy and the demand for faster printing operation.
The fixing devices resorting to the induction heating are broadly divided into two types. To one of the two types belong such low-frequency induction heating fixing devices as generate heat by winding a coil around a core (iron core) forming a closed magnetic circuit, disposing a conductive fixing roller in such a manner as to encircle the core enwrapped with the coil, and directly advancing an alternating current from a commercial power source, namely an alternating current of a frequency of 50-60 Hz, to the coil thereby causing the fixing roller to generate an induced electric current. To the other type belong such high-frequency induction heating fixing devices as generate heat by winding a coil around a core (iron core) optionally forming or not forming a closed magnetic circuit, disposing a conductive fixing roller in such a manner as to enclose the core enwrapped with the coil, and advancing a high frequency of several kHz--some hundreds of kHz to the coil thereby causing the fixing roller to generate an eddy current.
The fixing device using the induction heating as described above has a large effect in conserving energy and is capable of elevating temperature with only a small amount of electric power as compared with the halogen lamp. The low-frequency induction heating fixing device in particular obviates the necessity for a circuit adapted to generate a high frequency and allows the temperature of the fixing roller to be controlled at a virtually constant level by simply controlling the ON-OFF state of the alternating current applied from the commercial power source to the coil as compared with the high-frequency induction heating fixing device. It, therefore, is at an advantage in enjoying simplicity of the construction of the device and allowing the device to be manufactured at a low cost. It further enjoys a great energy conserving effect because it entails virtually no leakage of magnetic flux and allows efficient generation of an induced electric current in the fixing roller on account of the provision of the core forming a closed magnetic circuit.
While the low-frequency induction heating fixing device has such a great energy conserving effect as described above, it has various problems yet to be solved. One of the problems is that the adjustment of temperature becomes difficult because the induction heating by nature heightens the temperature increasing rate of the fixing roller. When the halogen lamp is used as a heat source, the temperature of the halogen lamp can be properly adjusted by measuring the temperature of the fixing roller with a thermistor and feeding back the result of this measurement because the rate of the temperature variation of the fixing roller is not very high owing to the thermal capacity of the halogen lamp itself and the thermal capacity of the fixing roller. In the case of the induction heating, the fixing roller itself generates heat as soon as the electric power is supplied to the coil and the rate of the temperature elevation of the fixing roller at times surpasses the speed of response of the thermistor. Especially when the temperature is elevated from the normal room temperature to the fixing temperature, the delay of the response of the thermistor possibly compels the fixing roller to yield to an excessive temperature elevation.
The fixing device utilizing high-frequency induction heating as disclosed in JP-A-07-287,471, for example, eliminates this problem by using for a fixing roller itself which is subjected to induction heating an element resembling a thermistor and controlling the high-frequency electric current applied to the coil in consequence of the elevation of temperature of the fixing roller.
This method is feasible solely because it concerns a high-frequency induction heating resorting to such a high-frequency control circuit as a converter which is capable of constantly applying a high frequency to the coil and controlling the frequency, the applied voltage, or the amount of electric current thereof. The method of the principle disclosed in the patent publication mentioned above, however, cannot be utilized when the temperature is adjusted, according to the feature of low-frequency induction heating, by switching the alternating current from the commercial power source without resorting to the use of a special control circuit as a power source.
Another problem of the low-frequency induction heating fixing device is that the induction heating, unlike the halogen lamp, does not permit direct discernment of the presence or absence of passage of electric current therethrough from the appearance. The halogen lamp continues to light so long as it is passing electricity, namely while it is kept hot by the passage of electricity. When the door of the copying machine is opened to inspect the interior for the purpose of maintenance, therefore, a look at the halogen lamp suffices to find whether or not it is lighting, namely whether or not it is passing electricity. The induction heating does not permit discernment of actual presence or absence of the passage of electricity because it has no light-emitting element of any sort. The same remarks hold good where the temperature of the fixing roller is detected by a thermistor, for example. While the fixing roller is being heated, the remaining heat thereof possibly compels the thermistor to manifest a state of high temperature even during an interruption of the passage of electricity. Thus, the question whether the warmth of the thermistor is caused by the heating due to the passage of electricity or by the remaining heat eludes a decision.
Further, the maintenance of the device which resorts to induction heating poses a new problem that since this device has no such a light-emitting element as a halogen lamp, how the trouble in the coil or the switching circuit serving to control the passage of electricity to the coil should be diagnosed is a query.